1. Field of the Invention
The present invention relates to an auto focus unit and a digital camera that adjusts a position of a focus lens in an optical system automatically so that an optical image of an object through the optical system can be brought into focus.
2. Description of the Related Art
A recent digital camera has an auto focus function. Several methods currently exist for carrying out the auto focus function, one of which is explained in brief below. A scanning-area is fixed upon the light-receiving surface of an imaging device to capture the optical image. Whenever the imaging device captures the optical image, a focus lens is re-positioned along an optical axis and a contrast value in the scanning-area is detected. The detected contrast values corresponding to a plurality of positions of the focus lens are recorded in a memory. One position of the focus lens, of which the contrast value is highest among the recorded contrast values, is selected as the in-focus position of the focus lens, and the focus lens is subsequently moved to this selected position. In the above method, it is necessary to compare a plurality of detected contrast values in the fixed scanning-area.
In addition, an image-pursuit function that enables the location of the image of a target-object to be tracked automatically is known. As the image of a target-object moves about within a space corresponding to an entire image to be captured, the image pursuit function pursues the movements of the image of the target-object about that space, crossing multiple, sequential time periods so that a plurality of the entire image to be captured is captured sequentially. A moving object in an entire image to be captured can be focused upon by carrying out the auto focus function in cooperation with the image-pursuit function. For example, the scanning-area capturing an optical image of a target-object is moved according to the movement of the target-object within the entire image by carrying out the image-pursuit function, and the auto focus function is carried out based on the moved scanning-area.
However, the auto focus function does not work well with a moving object if the optical image of the target-object is dark.